1. Field of the Invention
This invention relates to polyisocyanate compounds containing isocyanurate groups and more particularly to such compounds which are prepared by trimerizing aliphatic diisocyanates.
2. Description of the Prior Art
The trimerization of hexamethylenediisocyanate (HDI) using various catalysts has been known for a long time and is described in a great number of patents. (cf. Accepted West German Specification Nos. 10 13 869, 12 03 792, 22 26 191; Unexamined West German Application No. 26 16 415; British Pat. Nos. 952,931, 966,338; U.S. Pat. Nos. 3,211,703, 3,330,828).
None of the processes described in these publications permit the problem-free production in a technically simple manner of practically colorless, low-viscosity, solvent-free polyisocyanates which are derived from HDI and contain isocyanurate groups.
The technical production of the isocyanatoisocyanurates based on HDI was first described in Unexamined West German Application No. 28 39 133. The products obtained with this process exhibit a viscosity less than 10,000 mPas at 25.degree. C. and a NCO content of 18-24 weight %, especially from 20 to 23 weight percent, and represent valuable starting materials of the production of non-fading polyurethane varnishes.
However they have a disadvantage in that they have only a limited compatibility with the polyhydroxy compounds generally used in PUR varnishes and, furthermore, have a limited solubility in the frequently used non-polar solvents such as, for example, mineral spirits. Another disadvantage of these isocyanatoisocyanurates based on HDI is their only moderate thermal and oxidative stability, which becomes particularly obvious when lacquer films based on these compounds are overbaked.
Therefore a need has continued to exist for polyisocyanates containing isocyanurate groups based on aliphatic diisocyanates which are compatible with the polyhydroxy compounds conventionally used in PUR varnishes and which have good thermal and oxidative stability.